What we'd need would be a lot of data... fifty people, at least, recording every single dream as well as keeping diaries of what had happened to them and when, as well as a good record of major world events--probably for a year or more.

Then we'd need to find some way to find the odds of a random dream pointing to something in the diaries or world events; and if there were a significant percentage more precognitive dreams than chance would allow, then there'd be a sign that there's something more to precognition... Not necessarily something supernatural, mind you; the brain is a powerful computer. For example, your knowing that a 40-year-old woman doesn't normally break her hips, and perhaps knowing some information about your friend's grandmother, could've helped your brain put two and two together in her sleep.

It would be a big project, though. I don't know if it's ever been done; but even for the sheer utility of so many dreams from so many people collected in one place it would be worth it.